A large portion of degraded croplands around the world are undergoing land-use conversions, which aim to improve soil fertility of degraded croplands and to optimize the structure and function of ecosystems. Nevertheless, to better evaluate the effectiveness of cropland conversion, studying soil fertility only is not enough; more mechanistic features that are related to ecosystem structure and function should be considered.

Soil microbes play a key role in decomposing soil organic matter and, thus, they control soil biogeochemical cycles. However, the growth and activity of soil microbes are often limited by resources such as C, N, and P, which is referred to as microbial resource limitation. Previous studies have shown that cropland conversions can change soil C, N and P status; it is reasonable to believe that microbial resource limitation may be altered during cropland conversion. To date, however, our knowledge regarding how cropland conversions affect microbial resource limitation is still poor.

In a paper in Geoderma, researchers in the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) studied three common cropland conversion strategies in a karst area of southwest China (i.e., conversion with i) Toona sinensis (TS), ii) Guimu-1 hybrid elephant grass (GG), and iii) a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG) using cropland (CR) with maize-soybean rotation as reference. Enzymatic stoichiometry was calculated and used as an indicator of microbial resource limitation.

The team found that soil enzyme activity was altered after cropland conversions, which was strongly related to changes in soil properties. Enzymatic stoichiometry further showed that microbial carbon (C) limitation was mitigated under all three conversion strategies, but nitrogen (N) and phosphorus (P) limitations were aggravated under TS and GG. “The mitigated limitation of microbial C may be due to the elevated soil C content, and the aggravated microbial N and P limitation may have been due to increased soil C/N and C/P, respectively.” said CHEN Hao, a doctoral researcher at ISA and first author on the paper.

The findings suggests that cropland conversion can change microbial resource limitation, which should be considered in the assessment and management of rebuilding ecosystems after cropland conversions.

This work was funded by the National Natural Science Foundation of China (31872691, 41877094, and 31760153), Youth Innovation Promotion Association of CAS to Hao Chen (2019355), National Key Research and Development Program of China (2016YFC0502404), Natural Science Foundation of Guangxi Province of China (2017GXNSFAA198038), National High-level Talents Special Support Program to Dejun Li, and Guangxi Bagui Scholarship Program to Dejun Li.

The study entitled “Cropland conversion changes the status of microbial resource limitation in degraded karst soil” has been published in June 2019 in Geoderma. This article is available for download at https://doi.org/10.1016/j.geoderma.2019.06.018.

Contact: LI Dejun

E-mail: dejunli@isa.ac.cn

Institute of Subtropical Agriculture, Chinese Academy of Sciences

Effects of cropland conversion on indicators of microbial resource limitation (Image by Chen Hao)